1. Field of the Invention
This invention relates to a heating pad, more particularly to a heating pad having a phase change material.
2. Description of the Related Art
A therapeutic heating pad which is made of a phase change or thermal energy storage material typically includes an envelope 1 filled with a phase change material 10, as shown in FIG. 1. The phase change material is in a solid or gel-like format normal or room temperature. When the heating pad is to be used, it is put into water and heated so that the phase change material absorbs heat and is transformed into liquid, thus storing thermal energy. At this time, the heating pad can supply heat for therapeutic purposes. When the phase change material becomes cold due to its liberation of heat, the phase change material returns to its solid or gel state. While such a heating pad has a simple construction, because the heating pad must be heated in water, it is suitable only for the household use. The application thereof is therefore limited.
FIG. 2 shows another conventional heating pad 1A which contains a liquid state phase change material 11 enclosing a metal plate 12. When it is used, a force is applied to the metal plate 12 to deform the metal plate 12. The phase change material 11 which is in the liquid state begins to liberate heat and change into a gel-like form. This heating pad 1A is also simple in construction. However, when the heating pad 1A is to be reused, it must be heated in water to cause the phase change material 11 to return to its liquid form from the gel-like form. Therefore, the application of the heating pad 1A is also limited. It is thus desirable to develop a heating pad with a self-heating means to heat a phase change material contained therein.